1. Field of the Invention
This invention relates to the molding of thermoplastic materials, and more particularly to articles useful as molds for such materials.
2. Description of the Prior Art
Various types of molds have long been in use for preparing shaped articles from thermoplastic resins, in such operations as injection molding, blow molding, resin transfer molding, compression molding and casting. Molds for these purposes are typically manufactured from metal or a similar material having high thermal conductivity.
For most purposes, high thermal conductivity is desirable since it permits the resin in the mold to cool rapidly, shortening the molding cycle time. At times, however, cooling is so rapid that the resin freezes instantaneously at the mold surface upon introduction into the mold, forming a thin solid layer which, especially if it contains a filler, can create rough surfaces, voids and porosity.
There have recently been disclosed multilayer molds in which a metal core has an insulating layer bonded thereto, for the purpose of slowing the initial cooling of the resin during the molding operation. The insulating layer is fabricated of material having low thermal conductivity, thus slowing the cooling of the molded resin, and also having good resistance to high temperature degradation, permitting use in a mold maintained at high temperatures. Said layer is typically made of a resin such as polyimide, polyamideimide, polyethersulfone or polyetherketone, typically applied in uncured form (e.g., as a polyamic acid in the case of a polyimide or polyamideimide) and subsequently cured.
Resinous insulating layers have a major disadvantage, however: they are not mechanically strong and are easily abraded upon contact, for example, with filled thermoplastics. Thus, they do not have sufficient mechanical integrity to produce molded articles having surfaces of high quality.
A solution to this problem is disclosed, for example, in various applications such as commonly owned application Ser. No. 07/784,115, now abandoned. It consists of one or more skin layers of hard material, typically metal, bonded to the insulating layer. The skin layer may be deposited by such operations as electroless deposition, electrolytic deposition and combinations thereof.
Such deposition operations introduce their own problems into the mold fabricating process. It is well known, for example, that the adhesion of metal layers to resinous substrates is poor. This fact has dictated that the resin employed in the insulating layer be one which intrinsically has or can be modified to have relatively high adhesion to metal layers deposited thereon. One genus of resins having this property is the fluorinated polyimides, of the type prepared by the reaction of pyromellitic dianhydride with 2,2-bis[4-(4-aminophenoxy)phenyl]hexafluoropropane. Such polyimides are available from Ethyl Corporation under the trademark "EYMYD".
Fluorinated polyimides of this type may be subjected to various operations to improve their adhesion to metal. Methods for so improving adhesion are disclosed in other copending, commonly owned applications, notably Ser. Nos. 07/765,801, 07/874,448, now U.S. Pat. No. 5,290,597, 07/874,453, now U.S. Pat. No. 5,302,467, and 07/874,458, now U.S. Pat. No. 5,288,519. The major disadvantage in the employment of such resins and adhesion improving methods is that the resins are considerably more expensive than corresponding non-fluorinated polyimides and other resins of high thermal conductivity and stability at high temperatures, and the adhesion improving operation is an additional process step which may be burdensome and inconvenient.
A further problem is the difficulties involved in repairing a mold having a metal skin on a resin insulating layer. To repair even relatively minor damage such as one or more scratches which penetrate the metal skin, it is necessary to remove the mold from use and deposit a new metal layer thereon by further electroless or electrolytic deposition, often after removing at least the area of the old skin which surrounds the damage. Thus, the mold is out of service for a relatively long period, often several weeks.